1. Field of the Invention
The present invention relates to a vacuum pump and, more specifically, to a vacuum pump with blades for discharging gas molecules arranged on an inlet port side of the vacuum pump.
2. Description of the Related Art
Vacuum pumps are widely used in, for example, a device for discharging a gas in a chamber of an apparatus for manufacturing semiconductors to thereby bring the chamber into a vacuum state. Among these vacuum pumps, there are ones composed only of blades, ones composed of a combination of a blade portion and a screw groove portion, etc.
FIGS. 12 and 13 illustrate the structure of a conventional vacuum pump. FIG. 12 is a diagram showing a part of the top view of the pump, and FIG. 13 is a diagram showing a part of the section thereof.
This vacuum pump is provided with stator blades 50 fixed to a casing 10 that has an inlet port 16, and a rotor 41 having rotor blades 40 that are rotated while being fixed to a rotating rotor shaft 18. The respective stator blades 50 and the rotor blades 40 are arranged in the axial direction in multi-stages, and constitute an exhaust system 13 for taking from the inlet port 16 gas molecules A into a space between the rotor 41 and the casing 10 to discharge the gas molecules A.
Such vacuum pump accomplishes vacuumizing (exhaustion) by rotating with a motor the rotor shaft 18 at a high speed of ten to ninety thousand rpm under the normal state.
There is known a measure in which the outer diameter of the rotor blades 40 is increased in order to increase the peripheral speed of the rotor blades 40 and enhance the discharging ability. However, this causes a decrease in rigidity of the rotor blades 40, and hence the measure also includes enlargement of the inner diameter of the rotor blades 40. Due to this structure, while the gas molecules A enter with the same extent that the inlet port 16 has, the flow of the gas molecules A is interrupted in a dead space defined by the inner diameter of the uppermost stage of the rotor blades 40 facing the inlet port 16 (a space around the top of the rotor shaft 18) where there are no blades. The existence of this dead space is practically equivalent to a lowering of the effective area of the inlet port, which reduces the conductance as well as the amount of gas molecules entering into spaces between the rotor blades 40. This leads to a problem of decreased exhaust efficiency.
As countermeasures against that dead space in the center of the inlet port, there is proposed a vacuum pump in which a conic inducer 19 is attached to the upper end of the rotor shaft 18 as shown in FIG. 14. This proposed pump can give an outward motion component in radial direction to the gas molecules A that collide against the wall surface of the inducer 19.
However, the gas molecules A in a molecule flow region obey the law of cosines to head off in the normal direction with respect to the collision face, as shown in FIG. 14, and thus gains not only the outward motion component but also the upward (in the direction of the inlet port) motion component, resulting in an insufficient exhaust efficiency.
The present invention has been made to solve the above problems that the conventional vacuum pumps suffer from and, therefore, an object of the present invention is to provide a vacuum pump with enhanced exhaust efficiency achieved by increasing the amount of gas molecules that enter into spaces between rotor blades.
The present invention attains the above object through a vacuum pump comprising: a casing with an inlet port; a rotor shaft housed in the casing; an exhaust means arranged between the rotor shaft and the casing such that it can rotate together with the rotor shaft, the exhaust means discharging gas molecules, which are taken in through the inlet port, by rotating along with the rotation of the rotor shaft; and guiding blades arranged between the rotor shaft and the inlet port such that it can rotate together with the rotor shaft, the guiding blades imparting an outward motion component in radial direction to the gas molecules, which are taken in through the inlet port, by rotating along with the rotation of the rotor shaft.
According to the present invention, the guiding blades are formed on a forming surface that is formed into a conic shape the diameter of which is gradually decreased toward the inlet port.
According to the present invention, the guiding blades are formed such that the front thereof in the rotation direction is perpendicular to the forming surface.
According to the present invention, the guiding blades are formed such that the front thereof in the rotation direction is sloped down to the rear rotation direction with respect to the radial direction with the axis of rotation as its center.
According to the present invention, the exhaust means comprises at least a plurality of blades, and the number of the guiding blades is set by multiplying the number of rotor blades, which are arranged in the uppermost stage of the above plurality of blades, by its divisor or by an integer.
According to the present invention, the guiding blades are formed at positions corresponding to a casing""s decreased diameter portion on a casing inner wall the diameter of which is gradually decreased toward the inlet port.
According to the present invention, the exhaust means comprises a blade portion or a screw groove portion, or comprises a combination of the blade portion and the screw groove portion.